Ion implantation is a standard technique for introducing conductivity-altering impurities into a workpiece such as a wafer or other substrate. A desired impurity material is ionized in an ion source, the ions are accelerated to form an ion beam of prescribed energy, and the ion beam is directed at the surface of the workpiece. The energetic ions in the beam penetrate into the bulk of the workpiece material and are embedded into the crystalline lattice of the workpiece material to form a region of desired conductivity.
For arc discharge ion sources the outputs of some species (e.g. BF2+ ions from BF3 plasma and molecular ions from molecular feeds) are improved if the ion source is operated at cooler temperatures. To take advantage of this, various sources have been designed to improve source cooling. Implementations of cooler sources have relied on a variety of techniques. Some simply increase the cooling capacity of the cooling fluid in the ion source region (e.g. larger cooling channels, using higher rates of flow, reducing the temperature of incoming cooling fluid). Others reduce the thermal resistance between the heat production and heat sink regions of the ion source by moving the cooling closer to the plasma chamber (or by flowing coolant through the walls of the plasma chamber itself) so that a smaller temperature difference is required to drive a given heat flow. Still others reduce the thermal resistance between the heat production region and heat sink regions of the ion source by increasing the cross section through which the heat flows.
FIG. 1 shows a partial cutaway view of a portion of a conventional cooling arrangement for an ion source. An arc chamber 300 is thermally coupled to a cooling flange 302 of a cooling chamber 304 via a bar element 306. The bar element 306 is made from a metal or other material having desired conduction heat transfer characteristics. The cooling flange 302 may be coupled to a heat sink or a source of cooling fluid. As arranged, heat generated in the arc chamber 300 is conducted through the wall of the arc chamber, through the bar element 306, to the forward flange 302 of the cooling chamber 304. Heat is removed from the forward flange via the cooling fluid that may be circulated through or against the flange.
The effectiveness of such approaches is limited by the generally low thermal conductivities of the refractory materials which must be used in the ion source region, and the significant impediments to heat flow presented by the interfaces between different materials.
An additional limitation to such approaches, when using ion sources that are not dedicated to a single species, is that for some species (e.g., P+ ions from a PH3 feed and C+ ions from a CO2 feed) the output and lifetime of the source are improved by operating the source as hot as possible. The ability to switch between high and low temperatures would thus be advantageous. When the thermal resistance between plasma chamber and cooling surface has been lowered by mechanical changes frozen into the design this becomes difficult.
In view of the forgoing, it would be advantageous to provide a system and method for controlling the temperature of an ion source so that the operating temperature of the source can be cooled when using species whose outputs are improved by cooler temperatures, and so that operating temperature of the source can be allowed to rise when using species whose outputs are improved by higher temperatures.